You need a directional antenna. Take a look here http://www.radiolabs.com/Articles/wifi-antenna.html.
Just a base router will have issues going that far with any type of usable signal.
Basically since you know where your computer is in the other building, you get any router with an external antenna connection, get a directional antenna, and point it in the direction of your garage. A onmi-directional antenna like one built into regular home routers will disperse it's signal in more or less a circle instead of a single strong directional signal directed at a single point.